Often used in industrial measuring- and automation technology for measuring parameters of fluids flowing in a pipeline, not least of all also fluid dynamic parameters, such as e.g. a flow velocity and/or a volume flow rate, are measuring systems, which are formed by means of at least two mutually spaced, ultrasonic transducers, both externally mounted on a tube, or a pipe, as well as an operating- and measuring electronics electrically connected with each of the two ultrasonic transducers. Such measuring systems are described at length, for example, in German Patents, DE-A 10057188, DE-A 102007062913; US-A 2014/0366642, and US-A 2014/0123767; and Published International Applications, WO-A 03/098166, WO-A 2007/012506, WO-A 2009/156250, WO-A 2012/120039, WO-A 2013/029664, and WO-A 2013/079264; and also in the article published in IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 43, NO. 4, JULY 1996 entitled “Acoustic Transfer Function of the Clamp-On Flowmeter”, by Bernhard Funck and Andreas Mitzkus, and are, for example, also available from the applicant in the form of clamp-on, ultrasonic, flow measuring devices under the designations “Proline Prosonic Flow 91W”, “Proline Prosonic Flow 93P”, “Proline Prosonic Flow 93T”, and “Proline Prosonic Flow 93W”.
Measuring systems of the type being discussed comprise most often a straight, especially hollow cylindrical, tube, or pipe, having a lumen most often corresponding to a straight circular cylinder and surrounded by a tube, or pipe, wall composed, for example, of a steel, which tube, or pipe, is adapted to guide a volume portion of the fluid in its lumen, i.e. to be flowed through by the fluid. The tube, or pipe, can, in such case, be, for example, a tube, which is inserted into the course of a pipeline, or, for example, also, a pipe subsection of an already existing pipeline, consequently embodied as an integral part of such pipeline. Typical tube, or pipe, wall thicknesses lie, for instance, in a range from 2 mm up to several centimeters.
Each of the at least two ultrasonic transducers is placed, for example, namely secured or clamped, on an outside the tube, or pipe, wall facing away from the lumen of the tube, or pipe, in such a manner that, as a result, each of the ultrasonic transducers is acoustically coupled via the tube, or pipe, wall to fluid guided in the lumen. Furthermore, each of the ultrasonic transducers is adapted to convert an electrical voltage varying as a function of time into ultrasonic waves propagating through the tube, or pipe, wall and further through fluid guided in the lumen of the tube, and to receive ultrasonic waves propagating through fluid guided in the lumen of the tube, or pipe, and further through the tube, or pipe, wall and to transduce such into an electrical voltage varying as a function of time. The at least two ultrasonic transducers are additionally so mutually spaced, positioned on the outside of the tube, or pipe, wall, and so oriented that ultrasonic waves brought about by means of one of the ultrasonic transducers propagate within the fluid located in the lumen partially along a sound propagation path used as measuring path and thereafter acoustically couple via the tube, or pipe, wall into the other ultrasonic transducer.
The most often equally-constructed ultrasonic transducers are typically each formed by means of at least one piezoelectric transducer element, for example, composed of lead-zirconate-titanate (LZT_-Pb[ZrxTi1-x]O3) or some other piezo-ceramic, as well as by means of a coupling body sound conductingly coupled with the transducer element, for example, a coupling body of polyetherimide (PEI), which is embodied to provide a best possible, sound conducting, equally as well as long term stable, contact between ultrasonic transducer and tube, or pipe. In the case of an ultrasonic transducer used in a measuring system formed as a clamp-on, ultrasonic, flow measuring device, a center frequency lies, currently, typically in a range of, for instance, 0.5-6 MHz at a relative bandwidth, namely a 6 dB-bandwidth referenced to the particular center frequency, for instance, of 20-30% of the particular center frequency.
In the case of industrial measuring systems of the type being discussed, each of the two ultrasonic transducers is typically adapted to be operated in each case intermittently as a transmitter converting electrical power into sound power, and as a receiver transducing sound power into electrical power, i.e. the two ultrasonic transducers are provided to be operated alternately as a transmitter and as a receiver, in such a manner that always only, at most, one of the two ultrasonic transducers is transmitter. For effecting an electrical voltage varying as a function of time useful as a received signal of the ultrasonic transducer operated as receiver, the operating- and measuring electronics generates during operation of the measuring system, at least at times, a driver signal having a time variable, for example, even bipolar, electrical voltage for the other ultrasonic transducer operated, at the moment, as transmitter, for example, in such a manner that the particular driver signal is embodied for the one ultrasonic transducer, at least for a predetermined time interval, complementarily to the driver signal for the other ultrasonic transducer.
Measuring systems of the type being discussed, not least of all also measuring systems embodied as clamp-on, ultrasonic, flow measuring devices serving for measuring fluid dynamic parameters, ascertain the desired measured values often based on travel times (tAB, tBA), which ultrasonic waves propagating within the lumen of the tube require for traversing the measuring path in the particular sound propagation paths, especially based on travel time differences (tBA−tAB), namely based on differences between travel times (tAB) of ultrasonic waves propagating along the measuring path in a first measuring direction and travel times (tBA) of ultrasonic waves propagating along the measuring path in a second measuring direction opposite to the first measuring direction. For such purpose, the at least two ultrasonic transducers are most often so positioned and so oriented on the outside of the tube, or pipe, wall that the sound propagation path serving as measuring path has a main propagation direction inclined relative to an inner diameter of the tube by a beam angle (αF). A length (L) the measuring path corresponding to a path length of the sound propagation path corresponds, in such case, to a quotient of the inner diameter of the tube divided by the cosine of the beam angle (i.e. D/cos αF). The two ultrasonic transducers can be positioned, such as shown, for example, in the above mentioned Published International Application, WO-A 2013/079264, for example, on oppositely lying sides of the tube, or pipe, or, however, for example, also, such as shown, for example, in the above mentioned WO-A 2009/156250, respectively WO-A 03/098166, on an imaginary surface element of the tube, or pipe, spaced on the tube, or pipe, along the surface element, typically such that the sound propagation path includes a central region of the tube, or pipe, i.e. its lumen, such that a measured travel-time difference is proportional to an average flow velocity of the fluid. Used for measuring travel time in the case of conventional measuring systems are pulsed ultrasonic waves, namely ultrasonic waves in the form of wave packets having a limited number of oscillations. The wave packets, at times, also referred to as ultrasonic bursts, are generated intermittently with a predeterminable shot rate, which is most often held constant over a longer period of time, for example, in that the driver signal delivered by the operating- and measuring electronics has for the particular ultrasonic transducer a voltage embodied as a sequence of rectangular or sinusoidal voltage pulses (bursts) formed to pulse packets in a rhythm corresponding to the shot rate.
Taking into consideration the velocity of sound (cFL) in the fluid located in the lumen, for example, thus 1484 m·s−1 in the case of water at 20° C., as well as an instantaneous average flow velocity (V0) of the fluid, the travel times correspond with most often sufficient accuracy to the known formulas:
            t      AB        =          L                        c          FL                +                                            V              0                        ·            sin                    ⁢                                          ⁢                      α            F                                ,            and      ⁢                          ⁢              t        BA              =                  L                              c            FL                    -                                                    V                0                            ·              sin                        ⁢                                                  ⁢                          α              F                                          .      
Derived therefrom, the fluid dynamic parameters, average flow velocity (V0), and volume flow rate (QV), can be determined, for example, using the known formulas:
            V      0        =                  L                              2            ·            sin                    ⁢                                          ⁢                      α            F                              ·                                    t            ba                    -                      t            ab                                                t            ab                    ·                      t            ba                                ,            and      ⁢                          ⁢      Qv        =                            π          4                ·        K        ·                  D          2                ·                  V          0                    =                        π          4                ·        K        ·                  D          2                ·                  L                                    2              ·              sin                        ⁢                                                  ⁢                          α              F                                      ·                                            t              ba                        -                          t              ab                                                          t              ab                        ·                          t              ba                                            ,respectively. Also the velocity of sound characterizing the material of the fluid can be determined, for example, based on the formula:
      c    FL    =            L      2        ·                  (                              1                          t              ab                                +                      1                          t              ba                                      )            .      
The beam angle and, associated therewith, the path length are established, for example, by the orientation of the ultrasonic transducer relative to the pipe as well as by velocities of sound relevant in the measuring system for the sound propagation and by acoustic impedances of the measuring system. Knowing the actual structure of the measuring system and the velocities of sound (ci), and wave numbers (ki), of the utilized materials, including the fluid guided in the lumen of the tube, the beam angle can be earlier calculated in the form of a nominal beam angle (αF,nom), for example, using a beam acoustic model assuming planar wave fronts, based on Snell's law of refraction for acoustics, in order thereafter to be taken into consideration for ascertaining the measured value for the at least one parameter. Derived from the nominal beam angle (αF,nom), additionally also the path length can be nominally determined. Typically, the nominal beam angle and the nominal path length are established on-site, for instance, numerically ascertained in the course of a start-up of the measuring system, for example, based on the structure of the respective measuring system, data characterizing the system, as well as corresponding nominal material parameters for the tube, or pipe, and the fluid. In the case of clamp-on, ultrasonic, flow measuring devices for measuring fluid dynamic parameters of aqueous measuring systems, the nominal beam angle lies, for example, frequently at, for instance, 20°.
In order actually to achieve the high accuracy required for industrial measuring systems of the type being discussed, i.e. an accuracy with which the at least one parameter is to be measured, thus to be able to maintain correspondingly required, small measuring errors, besides a highly accurate measuring of the travel times of ultrasonic waves propagating along the particular measuring path, for example, also an exact as possible knowledge of the transfer behavior, i.e. the transfer function, of each of the ultrasonic transducers, the geometric dimensions of the tube, or pipe, as well as also the velocities of sound relevant for the measuring, and the wave numbers of the total measuring system, are required. Especially, it is, additionally, however, also necessary to assure that the beam angle (αF) actually established during the measuring in the measuring system corresponds as exactly as possible to the nominal beam angle (αF,nom) applied for calculating the measured values for the parameters. Thus, an angular deviation (ΔαF) existing between the nominal beam angle (αF,nom) and the actual beam angle (αF) should be as small as possible.
A special problem of measuring systems of the type being discussed lies, for example, in the fact that, such as, for example, also discussed in the above mentioned article in IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 43, NO. 4, July 1996 entitled “Acoustic Transfer Function of the Clamp-On Flowmeter”, by Bernhard Funck and Andreas Mitzkus, the tube, or pipe, has naturally a plurality of oscillation modes, in which the tube, or pipe, wall executes, or can execute, Lamb-waves, namely oscillations forming mixed pressure- and shear waves, in such a manner that the tube, or pipe, wall is deflected both in a radial direction as well as also in a longitudinal direction of the tube, or pipe (Lamb wave oscillation modes). These Lamb waves can be both symmetric waves (S0, S1, S2, . . . Sn) as well as also asymmetric waves (A0, A1, A2, . . . An). Most often, several of these Lamb wave oscillation modes can have resonance frequencies, which lie within the bandwidth of the respective ultrasonic transducer, consequently in the vicinity of its center frequency, i.e. within the bandwidth of the excited ultrasonic waves, wherein the actual resonance frequencies of the Lamb wave oscillation modes, or particular positions of their resonance frequencies in the frequency range, are regularly only earlier ascertainable approximately, for instance, based on the above indicated identifying data, i.e. the material parameters of the measuring system. Due to this situation, on the one hand, an exciting of a plurality of Lamb wave oscillation modes is practically unavoidable, on the other hand, it is, however, also not exactly foreseeable, with which intensity various Lamb wave oscillation modes will actually occur during operation. Consequently, it is earlier also not directly determinable, with which intensity and propagation direction the individual spectral fractions of the ultrasonic waves excited by means of the ultrasonic transducers will actually propagate through the lumen. As a result of this, the actually established beam angle can, even in the case of very slight deviations of the structure of a measuring system, namely deviations lying within usual tolerance limits, or very slight deviations of the material properties of the materials involved in the propagation of the ultrasonic waves, deviate from the respective nominal values significantly, namely from the above calculated nominal beam angle to an extent influencing the accuracy of measurement (or, inversely, the measuring errors) significantly, without that this can be detected in normal operation. As other influencing factors, further increasing the uncertainty concerning number and intensity of the actually excited Lamb wave oscillation modes, consequently the uncertainty concerning the deviation of the actual beam angle from the nominal beam angle, can be mentioned, for example, also temperature distribution within the tube, or pipe, wall, within the fluid, and within the ultrasonic transducer, as well as also the actual form of contact surfaces formed between each of the ultrasonic transducers and the tube, or pipe, wall.
In order to minimize the previously indicated disturbing influences of Lamb wave oscillation modes on the accuracy of measurement, in the case of some conventional measuring systems of the type being discussed, not least of all also in the case of conventional clamp-on, ultrasonic, flow measuring devices installed in industrial measurements technology, the driver signal is so generated by means of the operating- and measuring electronics, for example, by correspondingly adapted forming of the above mentioned pulse packets, or bursts, that, as a result, the particular received signal has a maximum signal power or at least achieves a predetermined minimum signal power. The required setting parameters for operating- and measuring electronics are most often ascertained by a corresponding tuning of the measuring system on-site, for instance, by an interactive aligning of the driver signal based on discrete Fourier-transformations (DFT), respectively discrete power spectral density (PSD) ascertained during start-up of the measuring system for the received signal. However, it has been found that, based on the criteria so far applied in conventional measuring systems for optimizing the driver signal, angular deviations (ΔαF) of essentially less than 0.4°, consequently relative measuring errors of significantly less than 2%, are currently scarcely implementable, or implemented.